Method of fabricating a steel part by powder metallurgy, and resulting steel part

ABSTRACT

A method for manufacturing by powder metallurgy a steel part is provided. A pre-alloyed powder is prepared, having the desired composition for said part, except for the O and N contents and optionally C contents, with O and N contents of at most 200 ppm, and with an Mn content from 0.4 to 2% by weight and a Cr content of less than or equal to 3%; the powder is placed in a container for which the walls define a space, the shape of which corresponds to that of the part, a getter being at the periphery of the powder, said getter has the capability at a high temperature of absorbing and reducing CO and absorbing nitrogen, and a vacuum is applied and the container is then sealed; the container and the powder are brought to a temperature causing sintering of the powder and densification of said powder not exceeding 5%, evolvement of nitrogen and CO from the powder and their absorption by the getter; densification of said powder is achieved by hot isostatic compaction in order to obtain said part; said part is separated from the container and from the getter; and peeling, heat treatment and machining of said part are achieved. The thereby produced steel part is also provided.

The present invention relates to metallurgy and more specifically to the manufacturing of steel parts by powder metallurgy.

BACKGROUND

It is known to one skilled in the art that micro-alloyed steels with manganese (their Mn content is of the order of 0.4 to 2%) and not very alloyed steels substantially without any chromium require control of their microstructure in order to guarantee good ductility, particularly in resilience. A microstructure having bulk areas of pro-bainitic ferrite is intrinsically brittle when a finer bainitic microstructure limiting the occurrence of this pro-bainitic ferrite is a necessary condition (but not sufficient condition) for guaranteeing good ductility after annealing. The parameters controlling the microstructure are chemical analysis, treatment temperatures (austenitization and annealing) and the quenching rate after austenitization.

The constitutive elements of vessels of nuclear power plant reactors are often made in a manganese steel of the type 16MND5 which fits the aforementioned definition, and for which the standardized composition (standard AFNOR 16 MND 5) is in weight percentages (as will be all the contents given in the text):

-   -   C≦0.22%;     -   Mn=1.15-1.60%;     -   P≦0.008%;     -   S≦0.008%;     -   Si=0.10-0.30%;     -   Ni=0.50-0.80%;     -   Cr≦0.25%;     -   Mo=0.43-0.57%;     -   V≦0.03%, being aware that for the parts to be coated, this         maximum content may be reduced to 0.01%;     -   Cu≦0.20%;     -   Al≦0.04%;         the remainder being iron and impurities resulting from the         manufacturing.

For this use, a steel of the A508 type (standard ASME SA-508/S/A-508M grade 3 is also used:

-   -   C≦0.25%;     -   Mn=0.5-1.00%;     -   P≦0.025%;     -   S≦0.025%;     -   Si≦0.4%;     -   Ni=0.4-1.00%;     -   Cr≦0.25%;     -   Mo=0.45-0.6%;     -   V≦0.05%;     -   Nb≦0.01%;     -   Cu≦0.2%;     -   Ca≦0.015%;     -   B≦0.003%;     -   Ti≦0.015%;     -   Al≦0.025%;         the remainder being iron and impurities resulting from the         manufacturing.

Usually, the elements of these vessels in 16MND5 are made by casting into ingots and then forging. Their mass may for some of them, attain several tens or even hundreds of tons.

SUMMARY OF THE INVENTION

It would be desirable to be able to make at least some of these elements, which may attain and exceed 10 tons, by powder metallurgy, so as to reduce the production time, to make the parts more easily inspectable and generally reduce cost related to their production.

However, tests have shown that parts made by powder metallurgy, even when these parts are of a relatively reduced size, did not give the possibility of obtaining satisfactory resilience levels, in spite of a relatively fine bainitic microstructure without any probainitic ferrite areas, for reasons which have not yet been clarified up to now. The problem is posed with even more acuteness for parts of large dimensions.

An object of the invention is to provide a method for making such parts, in particular parts of large dimensions, in 16MND5 steel and also in other steels and ferrous alloys for which comparable problems would be posed, by using powder metallurgy, this method nevertheless providing satisfactory mechanical properties to said parts, notably resilience at least equal to that obtained on casted and forged parts of the same composition, and having a microstructure of the bainitic type, as the one which is usually obtained on parts of the type mainly targeted by the invention.

For this purpose, a method for manufacturing by powder metallurgy a part in steel provided, wherein:

-   -   a pre-alloyed powder is prepared having the desired composition         for said part, except on the O and N contents and, optionally on         C, with O and N contents of at most 200 ppm, said powder having         an Mn content comprised between 0.4 and 2% by weight and a Cr         content of less than or equal to 3%;     -   the powder is placed in a container, the walls of which define a         space, the shape of which corresponds to that of the part to be         manufactured, a getter being positioned at least partly at the         periphery of the powder, said getter having the capability at         high temperature of absorbing and reducing CO and of absorbing         nitrogen by dissolution and the container is then evacuated and         then sealed;     -   the container and the powder which it contains is brought to a         temperature causing sintering of the powder and densification of         said powder not exceeding 5%, nitrogen and CO evolvement from         the powder and their absorption by the getter;     -   densification of said powder is achieved by hot isostatic         compaction by placing said container and the powder in a         pressurized chamber in order to obtain said part;     -   said part is separated from the container and from the getter;     -   and peeling, heat treatment and machining of said parts are         carried out for giving it its mechanical properties, its surface         condition and its desired exact dimensions.

Said part may be in a composition steel, in weight % after densification:

-   -   C≦0.25%;     -   Mn=0.5-1.60%;     -   P≦0.025%;     -   S≦0.025%;     -   Si≦0.4%;     -   Ni=0.4-1.00%;     -   Cr≦0.25%;     -   Mo=0.43-0.6%;     -   V≦0.05%;     -   Nb≦0.01%;     -   Cu≦0.2%;     -   Ca<0.015%;     -   B≦0.003%;     -   Ti≦0.015%;     -   Al≦0.04%;     -   O≦50 ppm, preferably ≦20 ppm;     -   N≦50 ppm, preferably ≦25 ppm;

the remainder being iron and impurities resulting from the manufacturing.

Said part may be a composition steel in weight %, after densification:

-   -   C≦0.22%;     -   Mn=1.15-1.60%;     -   P≦0.008%;     -   S≦0.008%;     -   Si=0.10-0.30%;     -   Ni=0.50-0.80%;     -   Cr≦0.25%;     -   Mo=0.43-0.57%;     -   V≦0.03%, being aware that for the parts to be coated, this         maximum content may be reduced to 0.01%;     -   Cu≦0.20%;     -   Al≦0.04%;     -   O≦50 ppm, preferably ≦20 ppm;     -   N≧50 ppm, preferably ≦25 ppm;

the remainder being iron and impurities resulting from the manufacturing.

Said part may be in a composition steel in weight %, after densification:

-   -   C≦0.25%;     -   Mn=0.5-1.00%;     -   P≦0.025%;     -   S≦0.025%;     -   Si≦0.4%;     -   Ni=0.4-1.00%;     -   Cr≦0.25%;     -   Mo=0.45-0.6%;     -   V≦0.05%;     -   Nb≦0.01%;     -   Cu≦0.2%;     -   Ca≦0.015%;     -   B≦0.003%;     -   Ti≦0.015%;     -   Al≦0.025%;     -   O≦50 ppm, preferably ≦20 ppm;     -   N≦50 ppm, preferably ≦25 ppm;         the remainder being formed with iron and impurities resulting         from the smelting.

Said getter may be in a material selected from titanium, zirconium, hafnium and alloys thereof, and stainless steel.

The getter may be in titanium or in a titanium alloy and the temperature of the powder during the sintering may be comprised between 950 and 1,065° C., preferably between 1,000 and 1,065° C.

The sintering and the densification by hot isostatic compaction of the powder may be carried out successively, without any intermediate cooling of the powder.

After having placed the powder in the space defined by the walls of the container, it is possible to subject it to cold isostatic compaction at a maximum temperature of 300° C. and under a pressure from 100 to 300 bars.

Said cold isostatic compaction may provide a reduction in the volume of the powder from 1 to 3%.

The wall of the container in contact with the powder may be made in the material making up the getter.

The getter may be a coating of the wall of the container.

The getter may form a separate part placed in the vicinity of the wall of the container in contact with the powder.

A steel part is also provided, characterized in that it was obtained by the method, and in that its oxygen content is ≦50 ppm, preferably ≦20 ppm, its nitrogen content is ≦50 ppm, preferably ≦25 ppm, and its cumulated oxygen+nitrogen content is ≦80 ppm, preferably ≦50 ppm.

As this will have been understood, embodiments of the invention are above all based on the observation by the inventors of the fact that the resilience problems encountered during the manufacturing of parts in 16MND5 by powder metallurgy came from too high oxygen and nitrogen contents in the initial pre-alloyed powder, and that removal of O and N during the sintering of the powder, if at least one of these elements was present in an excessive initial content, gave the possibility of solving these problems. It should be understood that the application of the method of embodiments of the invention goes beyond the sole manufacturing of parts in 16MND5 and other alloys of the same family, notably A508, and may relate to all iron alloys shaped by powder metallurgy, for which it would be found that the O and/or N contents in the initial powder would pose problems as to the properties of the final part. Steels containing from 0.4 to 2% of Mn and up to 3% of Cr in addition to Mn, form such alloys.

It is recalled that by <<pre-alloyed powder>>, is usually meant a powder for which the grains taken individually each have the targeted composition for the final part (except for modifications which may occur during the treatment of the powder). This pre-alloyed powder is defined as opposed to a powder which would consist of grains of diverse compositions and which, once mixed and sintered, would provide a part, for which the global composition would be the targeted one, but which may exhibit at a microscopic scale notable local composition differences.

The solution developed by the inventors notably consists of reducing these O and/or N contents before definitive densification of the pre-alloyed powder, by using a getter, i.e. a compound which, placed in the vicinity of the powder present in a container, will capture oxygen (in the form of CO) and/or nitrogen, because of its greater affinity for both of these elements than that of the powder to be treated.

The use of getters is well known when reduction of the oxygen content of the atmosphere surrounding a material (powder or other material) is desired during a heat treatment, in order to avoid contamination of the surface of the material with the oxygen of the ambient atmosphere. However, within the scope of the invention, the function of the getter goes much further, in that the inventors have ascertained that it was possible, under certain conditions, to also obtain deoxidation and/or denitridation in the bulk of the powder, even when the amount of powder applied adds up to hundreds of kilos, or even tons.

The getter thus becomes the main agent for a real metallurgical treatment of the powder, and it was surprisingly found that this treatment contributed to a highly substantial improvement in the resilience of the parts obtained after hot isostatic compaction (HIC) of the thereby treated powder and a heat treatment of the conventional type carried out on the part resulting from the compaction. Thus, one returns to the mechanical properties of the parts obtained by powder metallurgy at least equal to those of the parts obtained by casting into ingots and forging, notably their resilience.

The material making up the getter is preferably titanium, because of its relatively modest cost, and especially because of its remarkable capability of rapidly absorbing and in large amounts both oxygen and nitrogen (each several %). These elements are released from the powder as CO (from the reduction of the oxides of the powder by the carbon which is present therein initially) and molecular nitrogen. It will be seen later on why titanium should however preferably not be used if a temperature of 1,085° C. or more is contemplated for treatment of the powder.

As regards the amount of titanium to be used, the most important parameter to be considered is the ratio between the titanium surface area and the powder mass because of the strong absorption capacities of CO and N₂ by titanium. An order of magnitude of this ratio is from 4 to 20 cm² of Ti per kilo of powder, for a powder typically containing 100 ppm of oxygen and 120 ppm of nitrogen. The ratio will also have to be adjusted depending on the treatment time, which mainly depends on the dimensions of the part. The mass of Ti to be used is strictly speaking, dependent on the mass of powder to be treated and on the surface area of its grains, but experiment shows that a Ti sheet with a thickness of 0.5 mm positioned around the external surface area of the powder mass is sufficient for absorbing CO and nitrogen in the desired amounts for most of the cases. In any case, routine experiments give the possibility of checking whether the amount of titanium (or of any other material) used for forming the getter is sufficient, for obtaining absorption of the largest amount of CO and nitrogen as possible, for given treatment conditions. Experiment also shows that the transfers of CO and nitrogen towards the getter are carried out sufficiently rapidly and efficiently so that the treatment times required for deoxidation and denitridation of the powder are compatible with the requirements of industrial production.

Materials other than titanium may be contemplated for forming the getter. Zirconium and hafnium would have comparable actions, but their clearly higher cost makes them economically of less interest, especially for making parts with a high mass. Stainless steels may also be used, with the advantage of being able to support higher treatment temperatures than titanium, if the composition of the treated alloy makes such temperatures required or useful. But in their case, the kinetics of absorption of oxygen and nitrogen are substantially slower than for titanium, which is a serious drawback if several tons of powder have to be treated simultaneously. At least in the preferential case of making parts in 16MND5 for nuclear power plant vessels for which it is unnecessary to exceed a treatment temperature of 1,070° C., titanium and its alloys are in practice the most interesting materials for applying the invention.

If the initial content of the powder in a single one of the elements O and N is excessive, it is conceivable to use in order to form the getter, a material which would significantly only absorb this excess element and not the other one.

If the oxygen is present at a high content, for example of about 0.005 to 0.01%, in the initial powder, decarbidation of the powder should be expected, caused by the formation and the departure of CO. The decrease in the C content will be substantially equivalent in weight percentages to the decrease of the O content. Optionally this may have to be taken into account, i.e. provide that the final C content of the treated part will generally be less than that of the powder. It is therefore possible that a powder having a slightly too high C content at the start gives rise to a part for which the C content will be compliant with the requirements of the smelted grade. Conversely, a proper initial C content in the powder may no longer be appropriate on the final part if the decrease in the C content has led to having the C content strongly deviate from the requirements of the grade because the departure of CO during the treatment would have been excessive, because of the particularly high initial O content of the powder. Upon selecting the composition of the powder, it is therefore recommended to ensure a certain safety margin on the C content relatively to the targeted final content. This may be achieved by means of routine experiments and a good control by the provider of the powder, of the composition of the latter.

The container is adapted for giving the heap of powder, before its treatment, a shape and dimensions very substantially corresponding to those of the definitive part. Its geometry and its dimensions should be calculated following the rules of the art specific to the calculations of the containers for producing parts close to their final dimensions by hot isostatic compaction of pre-alloyed powders. After a hot treatment of the powder which leads to its sintering, the container which contains it is placed in a hot isostatic compaction chamber where complete densification of the part occurs. After this complete densification, all that remains to be done is to extract the part from the container and to treat it thermally in order to give it its definitive metallurgical structure, and to machine it in order to complete its dimensioning and its surface condition.

The parameters of hot isostatic compaction (HIC) and particularly the duration of the temperature and pressure plateau, are set according to the usual rules of the art for obtaining full densification and a homogenous temperature in all the points of the part, preferably one hour before the end of the plateau in order to ensure a sufficient error margin over the required time. Typical temperature durations under 1,000 bars are from 2 to 5 hours depending on the mass of the part.

Preferably, after putting the powder in the container and before its sintering, cold isostatic compaction is practiced, typically leading to a decrease in the volume of the powder by 1 to 3%, which corresponds to a pressure of 100 to 300 bars in the case of 16MND5 steel.

BRIEF SUMMARY OF THE DRAWINGS

The invention will now be described in more detail, with reference to the following appended figures:

FIG. 1 which schematically shows the various steps of an exemplary application of the method according to the invention;

FIG. 2 which shows the variations in the oxygen content as measured at the end of the treatment for different amounts of Ti used, added to the power mass during the manufacturing of slugs with the same geometry in a 16MND5 alloy of various compositions for containers with a diameter of 110 mm substantially containing 12 kg of sintered powder for 8 hours at 1,050° C. before HIC for 3 hours at 1,050° C. under 1,000 bars;

FIG. 3 which shows the variations of the nitrogen content as measured at the end of the treatment for different amounts of Ti used added to the powder mass during the manufacturing of slugs of a same geometry in a 16MND5 alloy of various compositions;

FIG. 4 which shows the resilience results obtained on different slugs of same geometry depending on their sole O content;

FIG. 5 which shows the resilience results obtained on different slugs with the same geometry depending on their N content alone;

FIG. 6 which shows the resilience results obtained on different slugs with the same geometry depending on their O+N sum; and

FIG. 7 which shows an example of a device for manufacturing slugs with relatively high masses, used for validation tests of the range of operating conditions for manufacturing industrial parts.

DETAILED DESCRIPTION

In the example which follows, the manufacturing of parts or blocks in steel with manganese 16MND5, will be specifically dealt with, being aware that the method of the invention may be applied to other iron alloys, more specifically to not very alloyed steels with manganese (i.e. containing 0.4 to 2% of this element) like the families of grades MC5, MND5, MSV5, MSV7, MV7, A508, CDV8, CDV9, and optionally up to 3% of chromium in addition to manganese. A Cr content of more than 3% would lead to the formation of Cr oxides which would not be affected by the method according to embodiments of the invention, and the latter would therefore not have the desired efficiency.

Generally, the invention finds a preferred application in the case of steels which would have the following composition, resulting from a compromise between 16MND5 and A508:

-   -   C≦0.25%;     -   Mn=0.5-1.60%;     -   P≦0.025%;     -   S≦0.025%;     -   Si≦0.4%;     -   Ni=0.4-1.00%;     -   Cr≦0.25%;     -   Mo=0.43-0.6%;     -   V≦0.05%;     -   Nb≦0.01%;     -   Cu≦0.2%;     -   Ca<0.015%;     -   B≦0.003%;     -   Ti≦0.015%;     -   Al≦0.04%;     -   O≦50 ppm, preferably ≦20 ppm;     -   N≦50 ppm, preferably ≦25 ppm;         the remainder being iron and impurities resulting from the         manufacturing.

As 16MND5 and A508 have similar compositions, the treatment conditions which have been described for 16MND5 would also be applicable to A508 with a benefit.

A container 1 is first prepared, which is for example in mild steel, consisting of two separable walls 2, 3 and defining between them, when they are assembled, a space 4, the shape of which corresponds to that of the part 5 which one desires to prepare.

A sheet 6 in titanium T40 with a thickness for example of 1 mm, is also prepared, forming the getter with for example a ratio of 10 cm² of Ti per kilo of powder, which is shaped so as to be able to be flattened against 1 2 of the walls 2, 3 which define the space 4, during the assembling of the container 1.

After this assembly, the shape and the dimensions of the space 4 very substantially correspondent to those of the part 5 which is desirably manufactured by powder metallurgy, by taking into account the shrinkage, (calculated elsewhere) which occurs during densification by hot isostatic compaction, as this is standard in this type of method.

Next, the space 4 is filled with the pre-alloyed powder 7 of 16MND5 steel intended to make up the part 5.

This powder typically has the composition:

-   -   C≦0.22%;     -   Mn=1.15-1.60%;     -   P≦0.008%;     -   S≦0.008%;     -   Si=0.10-0.30%;     -   Ni=0.50-0.80%;     -   Cr≦0.25%;     -   Mo=0.43-0.57%;     -   V≦0.03%, being aware that for the parts to be coated, this         maximum content may be reduced to 0.01%;     -   Cu≦0.20%;     -   Al≦0.04%;     -   Co≦0.1%;     -   Ti≦0.01%;     -   Nb≦0.01%;     -   Ta≦0.01%;         the remainder being iron and impurities resulting from the         manufacturing, notably from oxygen and nitrogen in variable         contents depending on the manufacturing conditions of the         powder.

As an example, this powder typically has an N content of 120 ppm and an O content of 100 ppm.

Preferably, after sealing the container in order to make it tight, the powder is degassed in order to remove air and humidity. This degassing, which is standard in operations for compacting powders, is carried out according to the rules of the art, for example by applying vacuum for 70 hours at a temperature of the order of 150° C.

The container 1 is then made air tight to the outdoor air, and the heat treatment is performed which will allow achievement of deoxidation and denitridation of the powder 7, by bringing the container 1 and the powder 7 to a suitable temperature for an adequate duration. Experiment shows that the diffusion of the gases evolving from the powder is sufficiently rapid so that they may attain without any difficulties the sheet 6. The grain size of the powder is without any significant importance, at least up to the usual millimetric grain sizes from the pre-alloyed commercial powders.

The treatment temperature should be selected according to the following criteria.

It should be sufficient for causing the reactions which will lead to departure of the oxygen (as CO) and of the nitrogen (in molecular form) from the pre-alloyed powder 7 and their capture by the getter 6, and for giving these reactions kinetics compatible with the imperatives of industrial production. It also causes sintering. But it should not cause any highly significant densification of the powder 7, in order to allow the gases CO and N₂ to circulate between the powder 7 and the titanium getter 6. In the described example, a minimum temperature of 950° C. or better 1,000° C. is recommended.

On the other hand, it is highly preferable that there be no interactions between the getter 6 and the powder 7 which would significantly cause for example diffusion of elements between them or a chemical reaction. Thus, in the described example, it is preferable to avoid exceeding the temperature of the eutectic Fe—Ti which is of the order of 1,085° C. Such an excess would have the drawback of polluting the surface of the future part 5, and therefore require machining over a depth greater than what would be desirable. Another substantial drawback of an excess of the temperature of this eutectic would be that the product of the eutectic reaction between titanium and the powder is extremely hard, and would only be able to be removed by long and costly grinding.

Generally selection of a maximum treatment temperature located below the eutectic of the main components of the getter 6 and of the powder 7 if there is one, will therefore be preferably selected, and with a sufficient safety margin in order to take into account the uncertainties on the temperature of the furnace and on the influence of the alloy elements on the exact temperature of the eutectic, therefore below 1,085° C. in the described example. For this example, a range from 950-1,065° C., preferably 1,000-1,065° C., may therefore be recommended.

The treatment time is essentially function of the thermal conductivity of the powder in its sintering state, on the amount of oxygen and nitrogen which has to be removed, and especially on the dimensions of the part 5 to be manufactured, notably its thickness, and on the surface area of the getter 6 added to the powder mass 7. The rapidity with which the targeted treatment temperature and the desired reactions and transformations notably depending on all of these parameters will be obtained in the whole powder 7. As an indication, this treatment time may typically be 8 hours for a cylinder with a diameter of 120 mm to 48 hours for a flat body with a thickness of 250 mm.

Because of the low conductivity of the powder, voids appear between the latter and the container. In order to avoid the possible problems generated by this mechanism (occurrence of voids, . . . ) two solutions may be contemplated, either through a continuous supply of powder, or through cold isostatic compaction if the latter is a pre-exquisite.

Performing, before sintering, a cold isostatic compression at a temperature of 300° C. at most and at a pressure from 100 to 300 bars, typically leading to a decrease in the volume of the powder by 1 to 3%, gives the possibility of dividing these sintering times by 2 or 3.

And then the method is continued by placing the container 1 in a hot isostatic compaction chamber 8, where it is proceeded conventionally with the possible completion of the sintering, and especially with the densification of the powder 7 under the effect of the pressure external to the container 1, in order to obtain the targeted part 5. The treatment temperature should, there again, be selected preferably for avoiding significant reaction between the getter 6 and the part 5 during compaction, and also for obtaining metallurgical structures for the part 5 compatible with the subsequent heat treatments. The pressure and the duration of the treatment are selected so as to obtain satisfactory densification of the powder 7 within a suitable time. A duration of the plateau at 1,050° C. under 1,000 bars is typically 3 hours for a flat part with a thickness of 250 mm.

Carrying out the sintering and the hot isostatic compaction successively in a same chamber equipped with heating means and pressurizing means may also be contemplated, by only pressurizing the chamber during hot isostatic compaction. This solution is economical from an energy point of view, since the treated metal does not substantially cool between both treatments, for which the temperatures are similar. Also, the handling operations of the container 1 are thus limited as far as possible. This same chamber may also be used during the optional cold isostatic compaction which precedes the sintering: it is then pressurized but the heating means are not activated, or then weakly so as to not exceed a temperature of 300° C.

Finally, the part 5 is taken out of the container 1 and separated from the getter 6. It undergoes peeling and machining for removing the remainders of the getter 6 after the heat treatment which follows.

As the temperatures maintained during sintering and densification are not adequate for obtaining the desired final metallurgical structure for the part 5, heat treatments carried out outside the container 1, therefore in the absence of the getter 6, should actually be carried out at any moment after separation between the part 5 and the remainders of the getter 6. Preferably, this is achieved before the final machining, if there is one, so that the latter may take into account possible deformations undergone by the part 5 during heat treatments.

A thermogravimetric study was conducted, spread out over 20 hours, of the nitridation and oxidation-carbidation of the T40 titanium which is a preferred material for forming the getter 6. It gave the possibility of determining:

-   -   that the nitrogen absorption kinetics by titanium only becomes         notable above 950° C.;     -   that the CO (and therefore oxygen) absorption kinetics by         titanium becomes notable as soon as 800-900° C.

Alternatively, slight cold isostatic compaction (CIC) may be carried out at a temperature of less than 300° C. of the powder 7 under a pressure for example of the order of 200 bars, in an optimal interval from 100 to 300 bars, after filling the space 4 and therefore before the denitridation and deoxidation treatment. This compaction may lead to a decrease in the volume of the powder of the order of 1% to 3% and gives the possibility of considerably improving the heat conductivity of the powder 7. The homogeneity of the targeted temperature for the following heat treatment, which will lead to deoxidation and denitridation of the powder, may thus be attained more rapidly.

By using the preceding method, cylindrical slugs with a diameter of 100 mm and weighing 12 kg and with a height of 180 mm (final dimensions after compaction and peeling) in 16MND5 alloy were made from powder batches with very comparable compositions, specified in Table 1.

TABLE 1 Compositions of the batches of powders used for the experiments O N initial initial C % Mn % Si % S % P % Ni % Cr % Mo % ppm ppm Batch 1 0.117 1.41 0.235 0.004 0.007 0.600 0.020 0.480 120 140 Batch 2 0.131 1.53 0.266 0.004 0.005 0.596 0.128 0.500 56 140 Batch 3 0.168 1.82 0.302 0.004 0.007 0.573 0.054 0.515 88 140 Batch 4 0.153 1.63 0.307 0.004 0.004 0.560 0.056 0.504 70 100

Moreover, all these batches had Cu contents of 0.012%, V contents of 0.010%, Al content <0.003%, Ti content <0.003%, As content <0.003%, Sn content <0.003%, and Ca content <0.0005%.

From these powders, 11 slugs were made according to the following operating procedure.

A container similar in its principle to the container 1 of FIG. 1 is prepared, which defines a space 4 with a cylindrical shape in order to give the powder 7 substantially the targeted dimensions for the slug. Against the wall of the container which defines the external surface of the space, a tubular getter in T40 titanium with a thickness of 1 mm is placed during tests conducted according to embodiments of the invention. The containers had substantially a diameter of 120 mm and a height of 200 mm and contained a little more than 12 kg of powder. The height of the getter tube is variable for each test according to the ratio between the titanium mass and the powder mass which is desired to be tested. The amount of titanium per kilo of powder is given, with the other experimental conditions in Table 2. The ratio between the surface area of the getter and the powder mass was varied from 10 to 34 cm² of titanium per kilo of powder. Reference tests were also conducted without any getter.

The container containing the getter (when there is one) is filled with powder in air, and then a vacuum is applied, maintained at 150° C. for 70 hours (in order to be sure to have degassed the powder, as this is customary in powder metallurgy), and finally sealed.

And then, for the treatments carried out according to embodiments of the invention, maintaining the whole at 1,050° C. is performed for 8 hours. It is essentially during this step that, if the getter is present, the O and N contents of the powder are lowered. At the same time, the powder undergoes sintering without any notable densification.

And then hot isostatic compaction (HIC) is carried out for densifying the slug, while placing the whole in a chamber pressurized to 1,000 bars, at a temperature of 1,050° C. for 3 hours.

After the compaction, the container and the remainders of the getter are removed by peeling, and the slug is cut into three cylindrical portions, with respective heights of 87 mm, 87 mm and substantially 5 mm. The portion with a height of 5 mm is used for characterizing the initial state of the slug. The two other portions are thermally treated by:

-   -   austenitization at 890° C. for 2 hours, and then oil-quenching         (OQ), at a quenching rate which is high, so that the resilience         of the sample which will then be measured will only depend on         its oxygen and nitrogen contents and not on an effect of the         microstructure; the high quenching rate limits the occurrence of         pro-bainitic ferrite, which is favorable to good resilience for         all the studied carbon contents;     -   and then annealing at 650° C. for 4 hours.

Table 2 summarizes the conditions of the different tests.

TABLE 2 Preparation and treatment conditions of the 12 kg slugs Ti/powder Slug Powder (cm²/kg) Sintering HIC Austenitization Annealing 1 Batch 1 34 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 2 Batch 1 11 and CIF 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 3 Batch 1 30 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 4 Batch 2 0 No 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 5 Batch 4 0 No 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 6 Batch 2 0 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 7 Batch 4 23 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 8 Batch 4 0 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 9 Batch 3 16 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 10 Batch 3 18 1050° C. 8 h 1050° C. 890° C. 2 h and 650° C. 4 h air 3 h OQ 11 Batch 3 0 Degassing 1050° C. 890° C. 2 h and 650° C. 4 h air 730° C. 50 h 3 h OQ

During the tests, the amount of Ti added to the mass of treated powder was therefore varied during tests. The results are visible in FIGS. 2 and 3 which respectively show, the oxygen and nitrogen contents measured at the end of the treatment for different amounts of Ti used added to the powder mass.

For the tests with references 4, 5, 6 and 8 having occurred without the presence of the getter in Ti, the O and N contents in the final slug did not decrease as compared with those present in the initial powder.

FIGS. 2 and 3 show that, while the oxygen and nitrogen contents of the sintered and densified slugs by cold isostatic compaction did not substantially vary, the use of a titanium getter as a sheet with a thickness of 1 mm lowers their contents, which may fall below 10 ppm for oxygen and below 20-30 ppm for nitrogen, for titanium amounts exceeding 10 g/kg (22 cm²/kg), with a saturation effect.

After densification by HIC, austenitization and annealing, these slugs underwent metallographic characterizations and mechanical tests.

The grain size is generally of 5 ASTM, with a few grains of 4 or 4.5 ASTM. This actually corresponds to standard requirements for 16MND5 used in the vessels of nuclear reactors. The structure is in majority bainitic annealed in every case.

Table 3 summarizes the results of various mechanical tests carried out, with the corresponding O, N and O+N contents of the slugs.

TABLE 3 Results of mechanical tests carried out on the slugs Kv Kv Rp_(0.2) Rm A Σ Kv −20° C. 0° C. 20° C. O N O + N Slug (MPa) (MPa) (%) (%) (J) (J) (J) (ppm) (ppm) (ppm) 1 464 578 24.5 76.3 243 241 224 45 50 95 470 582 25.6 77.1 199 248 232 2 513 618 22.5 69.5 39 64 88 120 140 260 518 624 21.3 68.7 34 72 92 3 508 611 23.6 78.0 79 197 243 6 3 9 504 604 22.1 78.4 173 192 249 4 653 734 20.1 65.9 28 33 58 95 90 185 654 736 19.7 66.8 25 37 58 5 629 729 19.3 64.4 23 26 37 90 120 210 632 734 19.6 63.8 23 24 52 6 594 677 18.3 67.3 40 60 66 58 110 168 644 733 18.1 66.0 28 53 49 7 627 733 20.6 72.2 134 157 193 38 40 78 621 730 20.6 71.9 138 143 182 8 694 796 16.7 58.6 24 27 33 99 140 239 704 811 17.3 58.3 27 27 33 9 540 652 21.0 76.0 47 90 137 36 29 65 535 649 23.0 76.0 65 92 190 10 520 634 26.8 74.8 103 172 196 3 50 53 532 640 24.7 74.6 160 177 201 11 522 637 25.0 75.0 53 53 71 88 32 120 532 642 22.0 75.0 47 78 80

The results of the tensile tests are satisfactory, both at room temperature and at 350° C. a usual temperature of use of nuclear reactor vessels. The ductility, expressed by the elongation at breakage A and necking Z, is compliant to said standard requirements. The elastic limits Rp_(0.2) and the tensile strength Rm are also or even better than usual for tensile strength. For each tested slug, the results obtained on both portions which have been thermally treated are shown. The deviations between the results obtained on both portions are generally small, of the order of the dispersions which would have been conventionally expected. The resilience of the slugs was especially tested, as, as this is seen, it is the characteristic which a priori would deter one skilled in the art from using powder metallurgy for manufacturing the parts which are mainly concerned by embodiments of the invention.

Measurements of resilience, illustrated by impact flexure Kv, were conducted at −20, 0 and +20° C. for the different slugs, having either been subject or not to the deoxidation and denitridation treatment of the original powder according to embodiments of the invention, the treatment conditions of which were given in Table 2 and the nitrogen and oxygen contents are given in FIGS. 2 and 3. FIG. 4 shows the results obtained according to only the O content, FIG. 5 shows the results obtained according to the N content alone, and FIG. 6 shows the results obtained according to the O+N sum. As the method according to embodiments of the invention simultaneously lowers the O and N contents of the powder, it is difficult to discriminate the effects of the contents of these elements on the sole basis of the tests carried out.

The Kv targeted at 0° C. is of at least 60 J for each sample taken individually. At −20° C., this minimum value is 28 J. At +20° C., this minimum value is 72 J. In FIGS. 4 and 5, the targeted values of minimum Kv have been reported for each sample.

FIG. 4 shows that these specifications are all observed when O is at most 40 ppm, and O contents of less than 20 ppm ensure systematically good to excellent values of Kv. On the contrary, O contents from 80 to 110 ppm, like in the initial powder, do not give the possibility of attaining the minimum Kvs required for sufficient safety margin.

FIG. 5 shows that comparable observations may be made for the N content. A content lowered to 40 ppm is often sufficient so that acceptable or even good Kv values are obtained, and contents of 25 ppm and less guarantee good results. The 90 to 140 ppm of the initial powder are on the other hand too high for reliably obtaining satisfactory Kv values.

Finally, if like in FIG. 6, reasoning is based on the O+N sum, there again a correlation is seen between the low content and a high Kv. A O+N value of 80 ppm at the most, ideally of less than 50 ppm leads to Kvs greater than the requirements laid down, with good reliability.

Such contents are made accessible by using a getter according to embodiments of the invention. They would not be directly accessible with other methods known for manufacturing powder, for example by atomization.

Generally, the Kv values obtained during tests are quite dispersed, since resilience does not only depend on the composition of the metal, but also on its microstructure, which the treatment conditions of the samples after sintering contribute to establish. In the present case, a bainitic structure is obtained on the product after sintering, and not a structure with probainitic ferrite which would have been less favorable. Nevertheless, all the tests corresponding to Table 1, 2 and 3 and to FIGS. 2 to 6 have been conducted for identical heat treatments leading to the same annealed bainitic final microstructure, the only difference lying in the composition of the powder (although the latter only varied within very low limits on the elements other than O and N) and either the use or not of a getter.

These tests clearly show that low O and N contents are, for reasons which further remain to be completely explained, conditions indispensable for obtaining the targeted resiliences, for identical treatment conditions and obtained metallurgical structure. From this point of view, the use of the method according to embodiments of the invention, considering the tests conducted on the slugs, appears as particularly advantageous, because of its efficiency and of its relatively reduced cost. The influence of the microstructure of the sample on the resilience has therefore been removed.

Sectional analyses of a getter 6 in a titanium T40 metal sheet with a thickness of 1.2 mm sampled on the slug (slug 10 of Table 2) have also been carried out after its use for 8 hours at 1,050° C., between its surface exposed to a 16MND5 steel powder 7 for which initially, the O content was 80 ppm and the N content 140 ppm, with a ratio of 20 cm² of Ti per kilo of powder. Its surface was in contact with the steel with 0.15% carbon making up the wall 2 of the container 1. They show:

-   -   that the distribution of the nitrogen is globally symmetrical         between both surfaces, with an increase in the nitrogen content         in the vicinity of each of the surfaces; this may be ascribed to         the fact that when the equilibrium pressure of nitrogen is high         and its absorption kinetics relatively slow, nitrogen diffuses         in titanium both from the powder 7 and from the wall 2 of the         container 1;     -   that the distribution of the oxygen is on the other hand clearly         asymmetrical: only the surface of the titanium which was found         facing the powder 7 has significantly absorbed oxygen; the         relatively low CO pressure and the rapid kinetics for reducing         CO and for absorbing oxygen ensures that CO does not circumvent         the titanium sheet 6 in order to attain its face located facing         the wall 2 of the container 1;     -   that the distribution of the carbon is apparently roughly         symmetrical, contrary to that of oxygen; the carbon         concentration is even slightly greater in the vicinity of the         face of the titanium sheet 6 which was facing the wall 2 of the         container 1; a portion of the carbon from the steel of the         container 1 diffuses in the solid state towards the titanium on         the one hand; absorption of the carbon after reduction of the CO         escaping from the powder is essentially carried out by forming a         surface layer of brittle TiC on the other hand; the latter         strongly tends to be detached from the titanium sheet 6 during         its handling operations between the experiment and the analysis:         the analysis of the sheet 6 therefore does not in fact account         for the reality of the absorption of the carbon from the powder         7;     -   that the nitrogen content of the Ti sheet 6 is of the order of         2% in the vicinity of its two surfaces, which is far from the         maximum solubility of nitrogen at 1,050° C. in titanium (6%);     -   that the O content of the Ti sheet 6 is of the order of 2.5% in         the vicinity of its surface area which was in contact with the         powder 7, which is far from the maximum solubility of oxygen at         1,050° C. in titanium (12%).

As the sheet 6 is far from being saturated with O and N at the end of the treatment, it is therefore unnecessary to provide that the sheet 6 has a relatively high mass with respect to that of the treated powder 7.

Tests for manufacturing tubular elements with the height of 287 mm, an inner diameter of 140 mm, an outer diameter of 370 mm, and therefore a wall thickness of 115 mm were also carried out. They were carried out with a powder belonging to batch 4 of Table 1 on a thick tube of austenitic steel (with a thickness of 30 mm). The Ti getter was placed on the internal wall of the container at the periphery of the cavity where the powder was placed, with a ratio of 8.6 g of Ti per kg of powder, representing a surface area of 18.2 cm' of Ti per kg of powder.

The method for filling the free space of the container with the powder was identical with the one applied to the previous 12 kg slugs. Maintaining the whole at 1,050° C. for 18 hours was first achieved for obtaining sintering. Hot isostatic compaction was then achieved under 1,000 bars at 1,050° C. for 3 hours. The final heat treatment of the tubular element extracted from the container and peeled consisted in austenitization, by maintaining 890° C. for 5 h followed by quenching in water at a rate estimated by modeling between 1.8° C./s (6,000° C./hour) at the periphery of the tube and 0.7° C./s (2,500° C./hour) in the core of the wall of the tube.

In all the cases, a final N content of less than 3 ppm and a final O content from 3 to 8 ppm on the tube were obtained. Very low contents are therefore obtained, and which do not clearly depend on the amount of getter used with respect to the powder mass. These results therefore tend to confirm that the influence of the getter/powder mass ratio is not preponderant, in any case for the manufacturing of massive parts. The getter surface area/powder mass ratio is a more crucial parameter, as well as the duration of the sintering treatment during which the essential part of the getter/powder reaction occurs. In spite of the low oxygen and nitrogen contents, the resiliences were particularly low in the areas with a low cooling rate during the quenching while they were good in the areas with a high cooling rate.

The manufacturing of two slugs with a diameter of 400 mm and a height of 210 mm was also carried out by means of the device illustrated in FIG. 7.

The container 8 is of a general cylindrical shape, with external dimensions of 400 mm in diameter and 234 mm in height (including the raised edges which guarantee good contact between the different constituents of the container 8). The thickness of the sheet which makes it up is 3 mm. It includes a bottom plate 9, a tubular side wall 10 and a lid 11 branched over a conduit 12 which is connected to a pump or equivalent so as to allow reduced pressurization of the inside of the container, after its filling and the sealing of the lid 11 on the side wall 10.

The getter in T40 titanium, when it is present, consists of three elements:

-   -   a planar sheet 13 which coats the bottom plate 9 of the         container 8, and which has a diameter of 375 mm and a thickness         of 1 mm, its mass is 0.5 kg:     -   a ring-shaped sheet 14 which coats the side wall 10 of the         container 8 over a portion of the height of the latter; the         sheet 14 has a diameter of 399 mm, a thickness of 1 mm and a         height of 95 mm; its lower edge 14 is placed at 57 mm from the         bottom plate 9 of the container 8; its mass is of 0.5 kg;     -   a planar metal sheet 15 which coats the lid 11 of the container         8, and includes an orifice 16 at right angles to the conduit 12;         it has a diameter of 375 mm, a thickness of 1 mm and a mass of         0.5 kg.

The powder used for preparing the samples is that of batch 3 of Table 1. An amount thereof of 147 kg is introduced into the container (including a relatively large amount, which may be representative of what would be the mass of certain of the industrial parts which are intended to be made with the method according to embodiments of the invention), which provides a Ti/powder mass ratio of 8.2 g/kg and a 18.2 cm' of Ti/kg of powder surface ratio.

Two slugs were produced with the same batch 3 of powder and substantially the same amount of titanium:

-   -   one with a sintering time of 16 hours     -   one with a sintering time of 48 hours     -   after sintering, the containers were densified by hot isostatic         compaction at 1,050° C. under 1,000 bars for a period of 3         hours.     -   after removing the container and the titanium getter by         machining, the slugs (diameter of 370 mm, height of 185 mm) were         heat treated with austenitization at 890° C. for 5 hours         followed by water quenching and annealing at 680° C. for 10         hours. The austenitization and annealing times were set         according to the dimensions of the slugs according to the rules         of the art. The water quenching caused a core cooling rate         estimated to be 0.8° C./s (3,000° C./hour), no doubt         insufficient for removing all the effect of carbon on the         resilience.

The microstructures were in majority bainitic for both slugs.

The results of the dissections of both slugs were the following:

For the slug sintered for 16 hours, the oxygen concentration varied from 5 to 90 ppm and that of nitrogen from 3 to 37 ppm in the bulk, the higher contents corresponding to the centre of the slug and at a maximum distance from the titanium getter. The resilience values were also highly variable ranging from very poor in the areas with strong concentrations of nitrogen and especially of oxygen to very good in the low oxygen and nitrogen areas.

For the slugs sintered for 48 hours, the oxygen and nitrogen concentrations were uniformly very low (3 to 5 ppm for oxygen and <3 ppm for nitrogen) and the Kv values from good to very good.

For the second slug manufactured by means of the device of FIG. 7, the treatment time was adjusted for guaranteeing that the totality of the powder volume has attained thermal equilibrium. This was not the case for the first slug, treated for only 16 hours.

The disappointing results of the first of both of these tests are ascribed to insufficient duration of the sintering before densification. The powder did not have the time to homogenize in temperature at at least 950° C. or better, 1,000° C., in all its volume. It therefore is only deoxidized and dinitrided only very partially before densification by hot isostatic compaction.

Between a slug made by means of the device of FIG. 7 and the tubular part described earlier, in addition to the quenching rate after austenitization, the analysis of the powder plays an important role on the final microstructure. This role is known to those skilled in the art. It is therefore necessary to adjust the analysis of the powder so as to guarantee the proper microstructure and from there the proper resilience in the totality of the volume of the part.

The successive steps of pre-sintering at 950-1,065° C. and of densification in this same range of temperatures may be carried out equally in the same chamber which will be only strongly pressurized during densification, or in different chambers therefore with the possibility that the container and the powder cool between both operations, possibly down to room temperature. The first solution is the most advantageous economically, notably from the point of view of the total energy consumption and the possibility of only using one facility for both operations.

The example which has just been described is particularly suitable for producing by powder metallurgy parts in 16MND5 manganese steel. However, as this has been said, this example is by no means limiting. Embodiments of the invention are applicable to the manufacturing of parts formed by other types of steels with an Mn content comprised between 0.4% and 2% and a Cr content of less than or equal to 3%, for which experience would show that the manufacturing by powder metallurgy with the obtaining of satisfactory mechanical properties notably resilience, would only be possible provided that the powder used only contains very small amounts of oxygen and nitrogen, comparable with those which have been determined during tests described earlier on the 16MND5 grade.

These amounts are an oxygen content ≦50 ppm, preferably ≦20 ppm, a nitrogen content ≦50 ppm, preferably ≦25 ppm, and a cumulated oxygen+nitrogen content ≦80 ppm, preferably ≦50 ppm.

As this has been stated, materials other than titanium and its alloys may optionally be used for making the getter 6. This may lead to modification of the temperature limits and of the duration of the treatments which have been given earlier, the essential point being that:

-   -   these temperatures are compatible with proper sintering of the         powder 7 of the steel used, which is not necessarily 16MND5;     -   and preferably, that they do not lead to the getter 6 and the         powder 7 being secured to each other during sintering, for         example by forming a eutectic between the iron and one of the         constituents of the getter 6, so that the separation between the         getter and the sintered part may be easily carried out and that         simple peeling is sufficient for removing from the surface of         the part the residues of getter which may have subsisted at its         surface.

It should be understood that the invention also provides a container 1 for fitting and hot isostatic compaction of a metal powder 7, interiorly coated over at least one portion of its surface which is intended to come into contact with the powder 7 of a getter 6 which generally has the capability of capturing, during heating the contaminants contained in the powder 7. By contaminants, are meant elements which may interfere with proper operation of the sintering with obtaining a part 5 having the sought mechanical and other properties. The oxygen and the nitrogen will often be the main contaminants to be captured. Notably oxygen may be captured, by a mechanism for reducing an oxidized gas such as CO evolving from the powder 7 and absorption of the thereby obtained oxygen.

Advantageously, for producing bimetal parts, for example tubes exteriorly or interiorly coated with a ferrule of a composition different from that of the tube, it may be provided that the container 1 includes one of the constituents of the bimetal part. The tube is then integrated into the container 1, and its coating is initially applied to it as a pre-alloyed powder, by means of the method according to embodiments of the invention. The adhesion of the coating on the tube is achieved by diffusion-welding during the treatment. In this configuration, the getter is placed on the surface of the container outside the diffusion-welding surface with the coating.

Generally, embodiments of the invention may in particular assume the following configurations:

-   -   the wall of the container in contact with the powder is         manufactured in the material making up the getter;     -   the getter (6) is a coating of the wall of the container (1);     -   the getter (6) is a separate part placed at the vicinity of the         wall of the container (1) in contact with the powder (7).

Embodiments of the invention have been essentially described in the case when the powder is a pre-alloyed powder of a steel slightly alloyed with Mn, and where the contaminants to be removed from the powder before its densification are O and N. But, as this was stated, the application of embodiments of the invention to the capture of other contaminants and to other types of steels containing from 0.4 to 2% Mn and from 0 to 3% Cr may be contemplated. 

1-20. (canceled)
 21. A method for manufacturing by powder metallurgy a steel part comprising: preparing a pre-alloyed powder having a desired composition for the part, except on the O and N contents and optionally on the C content, with O and N contents of at most 200 ppm, the powder having an Mn content comprised between 0.4 and 2% by weight and a Cr content of less than or equal to 3%; placing the powder in a container for which walls of the container define a space, a shape of the space corresponding to that of the part to be manufactured, a getter being positioned at least partly at a periphery of the powder, the getter having the capability, at high temperature, of absorbing and reducing CO and of absorbing nitrogen by dissolution, and a vacuum is applied and the container is then sealed; bringing the container and the powder contained therein to a temperature causing sintering of the powder and densification of the powder not exceeding 5%, evolving nitrogen and CO from the powder and their absorption by the getter; densifying the powder by hot isostatic compaction by placing the container and the powder in a pressurized chamber in order to obtain the part; separating the part is from the container and from the getter; and peeling, a heat treating and machining of the part for giving the part mechanical properties, a desired surface condition and exact dimensions.
 22. The method as recited in claim 21 wherein the part is in a steel of a composition, in weight % after densification: C≦0.25%; Mn=0.5-1.60%; P≦0.025%; S≦0.025%; Si≦0.4%; Ni=0.4-1.00%; Cr≦0.25%; Mo=0.43-0.6%; V≦0.05%; Nb≦0.01%; Cu≦0.2%; Ca<0.015%; B≦0.003%; Ti≦0.015%; Al≦0.04%; O≦50 ppm; N≦50 ppm; and the remainder being iron and impurities resulting from the manufacturing.
 23. The method as recited in claim 22 wherein O≦20 ppm.
 24. The method as recited in claim 22 wherein N≦25 ppm.
 25. The method as recited in claim 22 wherein the part has the composition, in weight %, after densification: C≦0.22%; Mn=1.15-1.60%; P≦0.008%; S≦0.008%; Si=0.10-0.30%; Ni=0.50-0.80%; Cr≦0.25%; Mo=0.43-0.57%; V≦0.03%, being aware that for the parts to be coated, this maximum content may be reduced to 0.01%; Cu≦0.20%; Al≦0.04%; O≦50 ppm; N≦50 ppm; and the remainder being iron and impurities resulting from the manufacturing.
 26. The method as recited in claim 25 wherein O≦20 ppm.
 27. The method as recited in claim 25 wherein N≦25 ppm.
 28. The method as recited in claim 22 wherein the part has the composition, in weight percent after densification: C≦0.25%; Mn=0.5-1.00%; P≦0.025%; S≦0.025%; Si≦0.4%; Ni=0.4-1.00%; Cr≦0.25%; Mo=0.45-0.6%; V≦0.05%; Nb≦0.01%; Cu≦0.2%; Ca≦0.015%; B≦0.003%; Ti≦0.015%; Al≦0.025%; O≦50 ppm, preferably ≦20 ppm; N≦50 ppm, preferably ≦25 ppm; and the remainder being iron and impurities resulting from the manufacturing.
 29. The method as recited in claim 28 wherein O≦20 ppm.
 30. The method as recited in claim 28 wherein N≦25 ppm.
 31. The method as recited in claim 22 wherein the getter is in titanium or in a titanium alloy, the temperature of the powder during sintering being between 950 and 1,065° C.
 32. The method as recited in claim 21 wherein the getter (6) is in a material selected from among titanium, zirconium, hafnium and alloys thereof, and a stainless steel.
 33. The method as recited in claim 21 wherein the getter is in titanium or in a titanium alloy, the temperature of the powder during sintering being between 950 and 1,065° C.
 34. The method as recited in claim 33 wherein the temperature of the powder during sintering is between 1,000 and 1,065° C.
 35. The method as recited in claim 21 wherein the sintering and densifying by hot isostatic compaction of the powder are successively carried out, without any intermediate cooling of the powder.
 36. The method as recited in claim 21 wherein after having placed the powder in the space defined by the walls of the container, the powder undergoes cold isostatic compaction at a maximum temperature of 300° C. and under a pressure from 100 to 300 bars.
 37. The method as recited in claim 36 wherein that the cold isostatic compaction provides a reduction in the volume of a powder by 1 to 3%.
 38. The method as recited in claim 21 wherein the wall of the container in contact with the powder is made in the material making up the getter.
 39. The method as recited in claim 21 wherein the getter is a coating of the wall of the container.
 40. The method as recited in claim 21 wherein the getter forms a separate part placed in the vicinity of the wall of the container in contact with the powder.
 41. A steel part obtained by the method as claim 21, the oxygen content being ≦50 ppm, the nitrogen content being ≦50 ppm, and the cumulated oxygen+nitrogen content being ≦80 ppm. 